1. Field of the Invention
The present invention relates to a process for forming an aggregate or agglomerate of fine glass particles on an outer surface of a starting material (e.g., on the circumference of a cylindrical starting material). More particularly, the present invention relates to a process for producing a glass preform for optical fiber, which comprises a glass material and deposit of fine glass particles formed on the circumference thereof. Such a glass preform may suitably be used as an intermediate product particularly in the fabrication of an optical fiber glass preform which is required to have high purity.
2. Related Background Art
Heretofore, the so-called "outside vapor-phase deposition process" as disclosed in Japanese Laid-Open Patent Application (KOKAI) No. 73522/1973 (i.e., Sho 48-73522), is known as a process for producing a silica glass (or fused silica) tube or preform for optical fiber. In this process, a glass tube is produced in the following manner.
Thus, fine particles of glass such as SiO.sub.2, which may be formed by hydrolysis of a raw material such as SiCl.sub.4, are caused to be deposited on the circumference of carbon or a refractory or fire-resistant starting material such as silica glass or alumina rotating about its horizontal axis as the rotational axis. The deposition of the glass is stopped after a predetermined amount of the glass particles are deposited, and the starting material is pulled out to provide a tubular glass aggregate. This tubular glass aggregate is sintered for transparency-imparting vitrification under a high temperature atmosphere in an electric furnace so as to provide a transparent tubular glass.
Alternatively, a glass tube or preform may also be formed in the following manner.
In such a process, a solid optical fiber preform (i.e., a preform having the inside completely filled up) may be used as a starting material in a similar manner as described above to provide a complex of the starting material and deposit of fine glass particles formed on the circumference of the starting material. Then, the complex is subjected, without pulling out the above-mentioned starting material, to heat treatment in a high temperature furnace so as to sinter the portion of the deposit of fine glass particles, whereby a transparent glass layer is further formed on the circumference of the optical fiber glass preform as the starting material.
In addition, Japanese Laid-Open Patent Application No. 186240/1986 (i.e., Sho 61-186240) discloses another process. In this process, as shown in FIG. 3, a glass raw material is supplied into flame 3 fed from a burner 2 for synthesizing fine glass particles to produce fine glass particles. The thus produced fine glass particles start to be deposited on the circumference of a substantially cylindrical or cylindrical tubular starting material 1 rotating about its axis (in a vertical direction) as a rotational axis 7, from the neighborhood of one end of the starting material 1. Then, the burner 2 is relatively moved in parallel with the axis of the starting material 1 so that deposit 6 of fine glass particles is formed around the circumference of starting material 1 with respect to the axis direction. Thereafter, the thus obtained deposit of fine glass particles is heated at a high temperature for vitrification to provide a transparent product. In FIG. 3, reference numeral 8 denotes a reaction vessel (soot deposition furnace) and reference numeral 9 denotes an exhaust port.
In the conventional processes as described above, however, there could occur a case such that foreign matter such as dust is attached to the starting glass material in the above-mentioned deposition process, and the attached foreign matter would cause a defect in the transparent vitrified glass product. Examples of such a defect may include a crystal and/or a bubble. In either case, a portion where the defect is caused cannot be used as a good glass product.
As a solution to the above problem, there is known a method wherein fine glass particles are deposited on a glass rod while the glass rod is purged with a clean gas, as disclosed in Japanese Laid-Open Patent Application No. 162646/1987 (i.e., Sho 62-162646). This method is effective to a certain extent. However, according to the present inventors' investigation, it has been found that the foreign matter such as dust could not be removed completely, when the glass rod is simply placed in a clean gas.